Organism enclosed within two shells.

Common Clams 
1. Shell less than one inch long; lateral teeh present on both sides of umbo (raised hump on shell, hump near hinge (Sphaeriidae)...2
1. Shell usually greater than one inch; lateral teeth posterior to umbo (when present) ...Unionidae
2. Umbo peak nearly median ...........3
2. Umbo peak close to one end..........Pisidium
3. Embryonic valves enlarged, usually separated from adult growth by a groove , appears like a cap on the umbo .... Musculum
3. Embryonic valves not distinctly enlarged or separated from the shell . . . . .    Sphaerium

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